1. Field of the Invention
Due to the high stability of methane, it has previously been processed through steam reforming steps in routes to desired chemicals. The process of this invention provides the direct synthesis of C.sub.2 hydrocarbons, such as ethylene, from methane by partial oxidation by electrocatalytic oxidative dimerization of methane in the anode compartment of a solid oxide fuel cell. The process of this invention using a solid oxide fuel cell is particularly suited for use in con]unction with the direct conversion of natural gas into DC electricity concurrently with the electrocatalytic partial oxidation of the methane to C.sub.2 hydrocarbon species C.sub.2 H.sub.4, C.sub.2 H.sub.6, and minor amounts of C.sub.2 H.sub.2.
2. Description of the Prior Art
The complete electrochemical oxidation of methane to CO.sub.2 and H.sub.2 O in the anode compartment of a solid oxide fuel cell, after its initial steam reformation to hydrogen, has been used in the conversion of natural gas into DC electricity. Handbook of Batteries and Fuel Cells, Ed. David Linden, 43-26 to 43-33, published by McGraw-Hill Book Company (1984).
The chemical synthesis of ethylene by oxidative coupling of methane using Sn, Pb, Sb, Bi, Tl, Cd, and Mn oxide catalysts is taught by Keller, G.E., and Bhasin, M.M., "Synthesis of Ethylene via Oxidative Coupling of Methane," Journal of Catalysis, 73, 9-19 (1982). However, the Keller, et al. article teaches Li, Mg, Zn, Ti, Zr, Mo, Fe, Cr, W, Cu, Ag, Pt, Ce, V, B, and Al oxides to have little or no such catalytic activity. The chemical synthesis of ethylene directly from methane in the presence of oxygen over LiCl-added transition metal oxide catalysts providing high selectivity and yield is taught by Otsuka, K., Liu, Q., Hatano, M. and Morikawa, A., "Synthesis of Ethylene by Partial Oxidation of Methane over the Oxides of Transition Elements with LiCl", Chemistry Letters, The Chemical Society of Japan, 903-906 (1986). Chemical partial oxidation of methane over LiCl--Sm.sub.2 O.sub.3 catalyst to C.sub.2 products, ethylene and ethane, with a high ethylene selectivity is taught by Otsuka, K., Liu, Q., and Morikawa, A., "Selective Synthesis of Ethylene by Partial Oxidation of Methane over LiCl--Sm.sub.2 O.sub.3," J. Chem. Soc., Chem. Commun., 586-587 (1986). Chemical conversion of methane to ethane and ethylene under oxygen limiting conditions over La.sub.2 O.sub.3 is taught by Lin, C., Campbell, K. O., Wang, J., and Lunsford, J. H., "Oxidative Dimerization of Methane over Lanthanum Oxide," J. Phys. Chem., 90, 534-537 (1986).
Oxidative coupling of methane over Ag and Bi.sub.2 O.sub.3 --Ag catalysts was carried out with oxygen electrochemically pumped through yttria-stabilized zirconia and it was found that the oxygen pumpted to the Bi.sub.2 O.sub.3 --Ag catalyst showed higher catalytic activity and selectivity for the production of C.sub.2 compounds compared to surface oxygen from the gas phase. Otsuka, K., Yokoyama, S., and Morikawa, A., "Catalytic Activity--and Selectivity--Control for Oxidative Coupling of Methane by Oxygen-Pumping through Yttria-Stabilized Zirconia," Chemistry Letters, The Chemical Society of Japan, 319-322 (1985). Electrochemical driving of O.sup.2- species through solid electrolyte yttria-stabilized zirconia decreased selectivity to C.sub.2 hydrocarbons and decreases the rate of production of C.sub.2 H.sub.4 using an Ag-Li/MgO catalyst electrode. Seimanides, S. and Stoukides, M., "Electrochemical Modification of Ag--MgO Catalyst Electrodes during Methane Oxidation," J. Electrochem. Soc., 1535-1536, July, 1986. Rare earth metal oxides Sm.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Y.sub.2 O.sub.3, and Bi.sub.2 O.sub.3 have been shown to have good catalytic activity and selectivity in chemical oxidative coupling of methane, Sm.sub.2 O.sub.3 being the most active and selective catalyst in the formation of C.sub.2 compounds. Otsuka, K., Jinno, K., and Morikawa, A., "The Catalysts Active and Selective in Oxidative Coupling of Methane," Chemistry Letters, The Chemical Society of Japan, 499-500 (1985).